1. Field of the Invention
The present invention relates to a vehicle steering mechanism and more particularly to a tiltable and telescopically extensible steering shaft mechanism for a vehicle steering mechanism.
2. Description of the Prior Art
A vehicle steering system having a tiltable and telescopically extensible steering shaft mechanism has been known for example by the Japanese laid-open patent application No. 58-33569. In the known type of the steering system, the steering shaft assembly includes an lower main shaft and an upper main shaft which are telescopically connected together through splines. The lower main shaft is supported at the lower end portion for tilting pivotable movements by a break-away bracket which is mounted on the vehicle body. The upper main shaft is held by a first upper bracket and the lower main shaft is further held at a position upwards the break-away bracket by a second upper bracket. A first driving device is provided between the first and second upper bracket to move the first upper bracket with respect to the second upper bracket. It will therefore be understood that through an actuation of the first driving device the upper main shaft is telescopically moved with respect to the lower main shaft to adjust the length of the steering shaft assembly.
The second upper bracket is mounted on the break-away bracket so that it can be moved in the shaft tilting direction with respect to the break-away bracket to produce a pivotable or a tilting movement of the lower main shaft. A second driving device is provided to drive the second upper bracket so that the lower main shaft is pivotably moved together with the first and second upper brackets and the first driving device.
In this known type of steering system, there are provided along the spline-connected upper and lower main shafts the first upper bracket, the second upper bracket supporting the first upper bracket and the break-away bracket supporting the second upper bracket. Further, there is provided between the first and second upper brackets the aforementioned first driving device which includes an electric motor for telescopically moving the upper main shaft with respect to the lower main shaft. There is also provided between the second upper bracket and the break-away bracket the aforementioned second driving device which includes an electric motor for tiltably moving the steering shafts. It is also necessary to provide an electric control circuit for controlling the operations of the electric motors.
The known arrangement is disadvantageous in that the driving device for the telescopic movements is located between the two movable brackets, namely, the first and second upper brackets. With this arrangement, an adverse effect has been experienced in the vibration absorbing characteristics of the steering system and vibrations has been transmitted from the vehicle body to the steering wheel.